Luminous efficacy is the amount of luminous flux in lumens that a particular lamp gives per unit consumption of electric power. The unit of luminous efficacy is in lumens/watt. Luminous efficacy is the measure of the energy efficiency of a lamp – and it varies based on the type of lamp.
The luminous efficiency of the incandescent lamp is around 10 – 20 lumens/watt whereas a fluorescent lamp is around 60 – 100 lumens/watt. This difference is due to fluorescent lamps being more energy-efficient than incandescent lamps. Currently LED lamps are coming into the market that has a luminous efficacy of up to 200 lumens/watt.
Correlated Colour Temperature
Correlated Colour Temperature (CCT) of a lamp is the temperature to which a black body if heated will emit the same color of radiation or light as that emitted by the lamp itself.
The Unit of CCT is Kelvin. Suppose the CCT of the fluorescent lamp is 4500K; this means that if a black body is heated to 4500K, it will emit the same color of radiation or light as that of the fluorescent lamp.
Based on CCT, lamps can be warm white, neutral white or cool white. If the CCT is less than 3000K, the lamp gives a yellowish-reddish color of light and this given a warm feeling to its surroundings. So lamps having CCT less than 3000K are termed as warm white.
If the CCT of any lamp is between 3000K and 4000K, then the lamp emits a white colour light and is known as neutral white.
If the CCT is more than 4000K, the lamp gives a whitish color of light which provides a cool feeling to its surroundings. So lamps having CCT more than 4000K are termed as cool white.
Colour Rendering Index
All objects have a particular color when viewed under natural light. If the same object is viewed under an artificial light source, the lamp reproduces the color of the object, but the color may or may not be the same as that under natural light.
Color rendering index (CRI) is the fraction with which the original color of the object is reproduced by the lamp. CRI of most lamps is less than 100%. Only incandescent lamps and halogen lamps have a CRI of 100.